1. Field of the Invention
The present invention relates to an electroluminescent display device, and more particularly, to an organic electroluminescence display device and a method of fabricating the same.
2. Discussion of the Related Art
In general, flat panel displays have been commonly used as display devices due to their thin profile, light weight, and low power consumption. Examples of flat panel displays include liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), and electroluminescent displays (ELDs). The electroluminescent displays may be categorized into inorganic electroluminescent displays (IELD) devices and organic electroluminescent display (OELD) devices depending upon source material for exciting carriers. The organic electroluminescence display (OELD) devices have high brightness, low driving voltage, and produce natural color images from the entire visible light wavelength range. In addition, the OELD devices have wide viewing angles and excellent contrast ratios because of their self-luminescence. Since the OELD devices do not require additional light sources, such as a backlight, the OELD devices have relatively small size, light weight, and low power consumption as compared with the LCD devices. Furthermore, the OELD devices may be driven by low voltage direct current (DC), and have short microsecond response times. Since the OELD devices are solid phase devices, they sufficiently withstand external impacts and have greater operational temperature ranges. In addition, the OELD devices may be manufactured at low cost. For example, only deposition and encapsulation apparatus are necessary for manufacturing the organic EL devices, thereby simplifying manufacturing processes.
The OELD devices may be classified into passive matrix-type and active matrix-type, depending upon a method for driving the devices. The passive matrix-type OELD devices do not have additional thin film transistors (TFTs), and are commonly used. The passive matrix-type OELD devices have scanning lines and signal lines that perpendicularly cross each other in a matrix shape. Since a scanning voltage is sequentially applied to the scanning lines to operate each pixel, an instantaneous brightness of each pixel during a selection period should reach a value resulting from multiplying the average brightness by the number of the scanning lines to obtain a required average brightness. Accordingly, as the number of the scanning lines increases, the applied voltage and current also increase. Thus, the passive matrix-type OELD devices are not adequate for high resolution display and large-sized areas since the devices easily deteriorate during use and power consumption is high.
Since the passive matrix-type OELD devices have many limitations in regards to image resolution, power consumption, and operational lifetime, the active matrix-type OELD devices have developed as next generation display devices for high resolution and large display area displays. In the active matrix-type OELD device, a thin film transistor (TFT) is disposed at each sub-pixel as a switching element that turns each sub-pixel ON and OFF. A first electrode connected to the TFT is turned ON/OFF by the sub-pixel, and a second electrode facing the first electrode functions as a common electrode. In addition, a voltage applied to the pixel is stored in a storage capacitor, thereby maintaining the voltage and driving the device until a voltage of next frame is applied, regardless of the number of the scanning lines. As a result, since an equivalent brightness is obtained with a low applied current, an active matrix-type OELD device having low power consumption, high resolution, and large area may be made.
FIG. 1 is an equivalent circuit diagram showing a pixel structure of an active matrix organic electroluminescent display device according to the related art. In FIG. 1, a scanning line 1 is arranged along a first direction, and a signal line 2 and a power line 3 that are spaced apart from each other are arranged along a second direction perpendicular to the first direction. The signal line 2 and the power line 3 cross the scanning line 1, thereby defining a pixel region “P.” A switching TFT “Ts,” i.e., an addressing element, is connected to the scanning line 1 and the signal line 2, and a storage capacitor “CST” is connected to the switching TFT “TS” and the power line 3. A driving TFT “TD,” i.e., a current source element, is connected to the storage capacitor “CST” and the power line 3, and an organic EL diode “DEL” is connected to the driving TFT “TD.” When a forward current is applied to the organic EL diode “DEL,” an electron and a hole are recombined to generate an electron-hole pair through the P (positive)-N (negative) junction between an anode that provides the hole and a cathode that provides the electron. The electron-hole pair has an energy that is lower than the separated electron and hole. Accordingly, an energy difference occurs between the recombination and the separated of the electron-hole pair, whereby light is emitted due to the energy difference. The switching TFT “TS” adjusts the forward current through the driving TFT “TD” and stores charges in the storage capacitor “CST.”
The OELD devices are commonly categorized as top emission-type and bottom emission-type according to a direction of the emitted light.
FIG. 2 is a cross sectional view of a bottom emission-type organic electro-luminescent display device according to the related art. In FIG. 2, one pixel region is shown to include red, green and blue sub-pixel regions, and first and second substrates 10 and 30 face and are spaced apart from each other. A peripheral portion of the first and second substrates 10 and 30 are sealed with a seal pattern 40. A thin film transistor (TFT) “T” is formed at each sub-pixel region “Psub” on an inner surface of the first substrate 10, and a first electrode 12 is connected to the TFT “T.” An organic electroluminescent layer 14 includes luminescent materials of red, green, and blue is formed on the TFT “T.” In addition, the first electrode 12 and a second electrode 16 are formed on the organic electroluminescent layer 14, whereby the first and second electrodes 12 and 16 apply an electric field to the organic electroluminescent layer 14. A desiccant (not shown) is formed in an inner surface of the second substrate 30 to shield from external moisture, and the desiccant is attached to the second substrate 30 by an adhesive (not shown), such as semi-transparent tape.
In the bottom emission-type OELD device, for example, the first electrode 12 functions as an anode and is made of a transparent conductive material, and the second electrode 16 functions as a cathode and is made of a metallic material of low work function. Accordingly, the organic electroluminescent layer 14 is composed of a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d formed over the first electrode 12. The emission layer 14c has a structure where emissive materials of red, green, and blue are alternately disposed at each sub-pixel region “Psub.”
FIG. 3 is a cross sectional view of a sub-pixel region of a bottom emission-type organic electroluminescent display device according to the related art. In FIG. 3, a TFT “T” having a semiconductor layer 62, a gate electrode 68, and source and drain electrodes 80 and 82 is formed on a substrate 10. The source electrode 80 of the TFT “T” is connected to a storage capacitor “CST,” and the drain electrode 82 is connected to an organic electroluminescent (EL) diode “DEL.” The storage capacitor “CST” includes a power electrode 72 and a capacitor electrode 64 that face each other with an insulating layer interposed between the power electrode 72 and the capacitor electrode 64, and the capacitor electrode 64 is made of the same material as the semiconductor layer 62. The TFT “T” and the storage capacitor “CST” are commonly referred to as array elements “A.” The organic EL diode “DEL” includes first and second electrodes 12 and 16 that face each other with an organic EL layer 14 interposed therebetween. The source electrode 80 of the TFT “T” is connected to the power electrode 72 of the storage capacitor “CST,” and the drain electrode 82 of the TFT “T” is connected to the first electrode 12 of the organic EL diode “DEL.” In addition, the array elements “A” and the organic EL diode “DEL” are formed on the same substrate.
FIG. 4 is a flow chart of a fabricating process of an organic electroluminescent display device according to the related art. At step ST1, array elements are formed on a first substrate that include a scanning line, a signal line, a power line, a switching TFT, and a driving TFT. The signal line and the power line cross the scanning line and are spaced apart from each other. The switching TFT is disposed at a cross of the scanning line and the signal line, while the driving TFT is disposed at a cross of the scanning line and the power line.
At step ST2, a first electrode of an organic EL diode is formed over the array elements. The first electrode is connected to the driving TFT of each sub-pixel region.
At step ST3, an organic electroluminescent layer of the organic EL diode is formed on the first electrode. If the first electrode is designed to function as an anode, the organic EL layer can be composed of a hole injection layer, a hole transporting layer, an emission layer, and an electron transporting layer.
At step ST4, a second electrode of the EL diode is formed on the organic EL layer. The second electrode is formed over an entire surface of the first substrate to function as a common electrode.
At step ST5, the first substrate is encapsulated with a second substrate. The second substrate protects the first substrate from external impact and prevents damage to the organic EL layer from any ambient air. A desiccant may be included in an inner surface of the second substrate.
The OELD device is fabricated through encapsulating the first substrate including the array elements and the organic EL diode with the second substrate. In addition, a yield of the active matrix OELD device depends on yields of the thin film transistor and the organic layer. Although the thin film transistor may adequately function, the yield of the active matrix OELD device varies due to impurities in the process of forming the organic layer to a thickness of about 1,000 Å. Accordingly, the yield of the active matrix OELD is reduced because of the impurities, and results in loss of manufacturing costs and source materials.
In addition, the active matrix OELD device is a bottom emission-type device having high stability and variable degrees of freedom during the fabrication process, but has a reduced aperture ratio. Thus, the bottom emission-type active matrix OELD device is problematic in implementation as a high aperture device. On the other hand, a top emission-type active matrix OELD has a high aperture ratio, and is easily fabricated. However, in the top emission-type active matrix OELD device, a choice of a material for the cathode electrode is limited since a cathode electrode is generally disposed over the organic layer. Accordingly, light transmittance is limited, and a luminous efficacy is reduced. Furthermore, in order to improve the transmittance, since a passivation layer should be formed in a thin film, air infiltration is not sufficiently prevented.